Sen. John Cornyn's emphasis on needing Specter to help deny Democratic senators a filibuster proof majority is an unexpected one.

WASHINGTON (CNN) - The Republican senator charged with overseeing the 2010 elections endorsed Sen. Arlen Specter for re-election in a letter sent to Pennsylvania Republicans last month.

The March 26 letter was made public Tuesday, one day after former Rep. Pat Toomey announced he would challenge Specter again for the Republican Senate nomination. Toomey, who is considered to be more conservative than Specter, unsuccessfully ran a primary campaign against the incumbent senator in 2004.

"As I survey the political landscape of the upcoming 2010 elections, it's clear we need more candidates that fit their states," Sen. John Cornyn of Texas, wrote in the letter. "While I doubt Arlen could win an election in my home state of Texas, I am certain that I could not get elected in Pennsylvania. I believe that Senator Specter is our best bet to keep this Senate seat in the GOP column.

"A vote for Arlen Specter is a vote for denying Harry Reid and the Democrats a filibuster-proof Senate," added Cornyn, who is the chairman of the National Republican Senatorial Committee.

Cornyn's emphasis on needing Specter to help deny Democratic senators a filibuster proof majority is interesting. Specter is considered by many to be a soft Republican vote in helping the GOP use the filibuster as a tool to block Democrats on some issues this year.

Cornyn also listed 13 bullet points in the letter that highlighted Specter's conservative credentials, such as the fact that he "voted for the Reagan and Bush tax cuts," and his decision to oppose the Employee Free Choice Act. The Texas senator also noted that the GOP faces a difficult electoral map next year, and having Specter as the GOP nominee will allow Senate Republicans to "focus our limited resources on growing the party and beating Democrats."

The letter was given to Specter and his campaign has the ability to distribute it, an NRSC aide said.